This finding, published Wednesday in Environmental Science & Technology, explains why methylated mercury, a neurotoxin, is produced in areas with no previously identified mercury-methylating bacteria. Methylmercury—the most dangerous form of mercury—damages the brain and immune system and is especially harmful for developing embryos. Certain bacteria transform inorganic mercury from pollution into toxic methylmercury.

The discovery also validates the recent finding that two genes are essential for the methylation of mercury. Previously, only a narrow range of microbes were recognized as mercury methylators, said co-author Dwayne Elias of the U.S. Department of Energy laboratory’s biosciences division.

“We showed for the first time that many different types of bacteria are able to produce this potent neurotoxin,” Elias said. “The newly identified microbes include methane-producing organisms that live in rice paddies, anaerobic wastewater treatment plants, northern peat lands and possibly within our bodies.”

A bacterium called Desulfovibrio desulfuricans, one of many that can transform elemental mercury into methylmercury. (Oak Ridge National Laboratory)

Elias and colleagues are testing a bacterium from the human intestine that they predict will also methylate mercury. Other bacteria able to transform inorganic into organic mercury include those used in biological dechlorination and metal-treatment systems. All of these organisms are anaerobic, which means they grow in habitats without oxygen, including aquatic sediments and wetland soils.

By identifying these organisms, the researchers may have explained why methylmercury is accumulating in unexpected places. The discovery may also help clarify how methylmercury is produced in the open ocean, according to co-author Cindy Gilmour of the Smithsonian Environmental Research Center. Gilmour noted that the newly identified fermentative bacteria may be common in the low-oxygen zones of the ocean where methylmercury production occurs. Ocean fish are the primary source of methylmercury in human diets worldwide.

Gilmour and Elias believe this work will have far-reaching implications for understanding the global mercury cycle. Scientists can now use the mercury-methylating organisms and their genes as global biomarkers and develop detection techniques to assess and eventually predict the extent of methylmercury production in a given environment.

“Based on the substantially expanded diversity of organisms and environments for methylation, a number of important habitats deserve further attention,” Gilmour said. “We now have a way to identify and track the abundance of mercury-methylating organisms within these environments, and we hope that the information can be used to devise strategies to minimize methylmercury production.”

Mercury is a global pollutant, released to the atmosphere through coal burning and other industrial uses and through natural processes. Most of the harm comes from methylmercury bioaccumulation, which is the buildup of the element in tissue that occurs when moving up the food chain.

The other authors of the paper, titled “Mercury methylation by novel microbes from new environments,” are Mircea Podar, Richard Hurt, Steven Brown, Anil Somenahally, Alex Johs and Kathryn Bailey of Oak Ridge National Laboratory, Allyson Bullock of the Smithsonian Environmental Research Center and Andrew Graham of Grinnell College. Funding was provided by the Department of Energy’s Office of Science, the National Science Foundation and the Smithsonian Postdoctoral Fellowship Program.

The Smithsonian Environmental Research Center has studied the science of Chesapeake Bay and coastlines around the globe since 1965. Its ecologists research invasive species, track blue crabs and other vital fisheries, study harmful chemicals and uncover the impacts of climate change. Today it houses some of the longest-running environmental field studies in the world. For more information, visit www.serc.si.edu.

UT-Battelle manages Oak Ridge National Laboratory for the Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit http://science.energy.gov/.